Thermal barrier coatings of alloyed yttria stabilized zirconia are used for many applications in modern turbine engines. A preferred method of producing the TBC is by thermal spraying a yttria+zirconia powder of the proper composition in order to produce a coating of the necessary composition, microstructure, and phase structure. Such powders are generally referred to as YSZ powders. The conventionally preferred types of powders used are spray dried powders agglomerated from separate particles of yttria and zirconia which are then densified by high temperature processing or powders formed by fusing yttria and zirconia and then crushing the alloyed material.
Spray drying is a well-known method of powder agglomeration used to produce food and pharmaceutical products in addition to thermal spray (TS) powders. The key attributes of spray dried products for the TS industry are that they are spherical, have a higher surface area compared to fused and crushed powders, and have low density. This results in powders that offer the benefits of being free flowing and that melt well within conventional thermal spray equipment.
Typically, as in the case of stabilized ceramics for TBC applications, fine particles (such as those having an average particle size of less than about 10 microns) of yttria and zirconia are mixed in water with organic binders and suspension agents to form a slurry. This slurry is then spray dried to create agglomerated particles that can then be applied using various thermal spray techniques to coat an object A shortcoming in the use of separate yttria and zirconia raw materials is that it may result in a chemical inhomogeneity of the powder particles. Even though the bulk concentration of the starting slurry may be correct, various factors such as relative particle size distributions, mixing methods, settling, and spray drying methods among others, may lead to non-uniform distribution of the constituents in the spray-dried particles
In order to achieve phase stability, the final coating must contain an alloy of yttria and zirconia. In the case of spray-dry powders, the alloying takes place during the process of thermal spraying or by additional processing of the powder after spray drying but before thermal spraying.
Alloying during the spraying process can be accomplished by applying the powder using a high-temperature thermal spray gun, such as a plasma gun, to ensure that the powder is melted and, by selecting the proper spraying environment to ensure that the powder has time to alloy prior to the cooling that occurs when the powder impacts the surface to be coated. A problem with alloying during thermal spraying is that alloying may not be consistent due to inhomogeneity of the powder, insufficient heat or residence time during the spray process, or variations in the spray process. In addition, non-uniform distribution of the constituents in the spray-dried particles can result in variations in the composition of the applied coating. Further, spray dried powders with individual yttria and zirconia particles above 10 microns in size will also be difficult to alloy. This non-uniform distribution and treatment of the powder may result in non-homogeneous microstructures in the applied coating which have poor or at least inconsistent thermal and mechanical cycling performance.
As a result, even though the previously described spray-dried powders offer certain advantages in thermal spraying, these advantages may be offset under some circumstances by the inconsistent composition and alloying treatment of the particles during spraying.
The need to alloy the powder during thermal spray can be eliminated by performing the alloying step prior to thermal spraying. Conventional techniques to achieve this rely upon plasma densification or sintering of the spray dried powder. One such plasma desnsified powder is currently available as Sulzer Metco 204CNS. This powder is also generally known in the industry as a HOSP powder. Such pre-processing eliminates the variations in the alloying caused by inconsistent treatment of the particles during the thermal spray process. The pre-processing also results in a more structurally stable powder that reduces powder breakdown prior to thermal spray that could prevent the proper alloying of the powder during spraying.
However, this method does not prevent inconsistencies that result from compositional inhomogeneity of each powder particle. The use of plasma densification or sintering adds significant cost to the production of the powder and this processing step is still subject to the inconsistencies in the distribution of the individual particles from the spraydrying. As a result, such powders may still produce coatings with inconsistent properties.
As an alternative to spray-dried and pre-processed powders, fused and crushed powders have been used in the area of thermal spray for TBC applications. Individual yttria and zirconia powders are mixed and fused using an induction arc or other process to produce a briquette of fused material. The briquette is then crushed to produce powder of the desired size suitable for thermal spraying, generally between 11 and 150 microns.
Fused and crushed powders exhibit angular, irregular morphologies. As a result, use of these powders can cause inconsistent powder feeding. In addition, the particles are generally denser and harder to melt. This results in lower deposition efficiency due to insufficient heating of the particles in the thermal spray jet.